In the preparation of polymeric foams or the like, significant advances have been made with the introduction of systems for mixing molten resin with blowing agents--said various normally gaseous or volatile liquid components--under high pressure. Pressures of at least about 3500 p.s.i.g. (pounds per square inch gauge) are typically required to ensure that the molten resin and blowing agents are suitably mixed. Extrusion of the resulting molten mixture into a low pressure zone results in foaming of a thermoplastic extrusion mass, by vaporization of the blowing agents. After a typical extrusion foaming step, the extruded material is ordinarily aged and then is thermoformed into containers and the like.
A variety of normally gaseous or volatile liquid blowing agents are used with olefinic or styrenic polymers. Representative blowing agents are common atmospheric gases (e.g. nitrogen and carbon dioxide) and hydrocarbons, including halogenated hydrocarbons (e.g. the C.sub.4 -C.sub.6 alkanes and chloro-fluoro methanes and ethanes).
Because carbon dioxide costs less than hydrocarbon blowing agents, it is economically advantageous to dilute hydrocarbon blowing agents with carbon dioxide. Use of carbon dioxide is also desirous, because during aging, blowing agents can escape into the atmosphere. The potential atmospheric pollution caused by the release of the blowing agents, in particular, by the release of certain halogenated hydrocarbons has led those in the industry to seek blowing agents comprised largely or entirely of non-polluting gases. Carbon dioxide is particularly beneficial because it is safe for food contact and is extensively used for direct contact freezing of food stuff.
Unfortunately, the extreme volatility of normally gaseous materials, such as carbon dioxide, has posed considerable problems in controlling the foaming process. Lack of proper control results in surface defects and corrugations in the extruded sheet material.
In an attempt to overcome these control problems, systems have been proposed for injecting a mixture of alkane liquid and carbon dioxide liquid into a molten extrusion mass, in a continuous extruder unit. U.S. Pat. No. 4,344,710 to Johnson et al. discloses one such system. The system proposed by Johnson et al. utilizes fluid handling means for pumping a plurality of diverse volatile liquids, including carbon dioxide, from a liquid source to the extruder means. A storage means maintains liquefied carbon dioxide under pressure. Heat exchange means connected to the storage means cools the liquefied carbon dioxide to prevent flashing thereof during pumping. A pump connected between the cooling means and the extruder means increases the pressure of the first stream to a level higher than the elevated pressure of the extruder, where it is combined with a pressurized stream of a second liquid blowing agent. The pump increases the pressure from a storage pressure ranging between 50-75 atmospheres (approximately 750-1125 p.s.i.g.), to an elevated injection pressure of about 340 atmospheres (approximately 5100 p.s.i.g.).
Such extremely high pressures are used in order to maintain the blowing agents in a liquid state and adequately control the mixing process. To maintain such high pressures, however, is expensive, difficult and hazardous. In addition, the system proposed by Johnson et al. is manually controlled, which substantially affects the accuracy of the ratios of the components of the extrusion mass.
A need thus exists for an improved apparatus which can blend a plurality of diverse, volatile liquid blowing agents at lower pressures which are less hazardous and can more efficiently and accurately control the ratio of the blowing agents.